I. Field of the Invention
The present invention relates to wireless communication. More particularly, the present invention relates to feedback controlled power supplies and to a novel and improved method and apparatus for controlling the output voltage of a Switch Mode Power Supply (SMPS) that is used as an input voltage to subsequent Low Drop Out (LDO) linear voltage regulators such as used within a mobile phone.
II. Description of the Related Art
Wireless communication networks rely on the ability of multiple user terminals to provide sustained high quality communication. In the case of wireless telephone networks the mobile units must be capable of sustaining high quality voice and data transmission. The design requirement of sustained high quality voice and data transmission must be weighed against the design requirements of battery operation, small size, low cost, and high reliability.
A mobile unit or wireless phone integrates numerous complex circuits. An RF transceiver is used to provide the wireless communication link with base stations. The RF transceiver is comprised of a receiver and a transmitter. The receiver receives the RF transmission from the base station via an antenna interfaced to the mobile unit. The receiver amplifies, filters, and downconverts to basedband the received signal. The baseband signal is then routed to a baseband processing circuit. The baseband processing circuit demodulates the signal and conditions it for broadcast through a speaker to the user.
User input via keypad presses or voice input to a microphone is conditioned in the baseband processing circuit. The signal is modulated and routed to the transmitter. The transmitter takes baseband signals generated at the mobile unit and upconverts, filters, and amplifies the signal. The upconverted RF signal is transmitted to the base station through the same antenna as used for the receiver.
Frequency synthesizers are used to generate the local oscillators required to perform the downconversion in the receiver and the upconversion in the transmitter.
The requirement that a mobile unit operate under battery power presents various issues that must be addressed. Batteries can only provide a limited amount of energy. Conservation of power consumption is the only way to extend battery life. Battery voltage varies considerably over its useful life and varies a moderate amount due to the load placed upon it. When multiple batteries are used in series this problem is only compounded. Additionally, specific components within the mobile unit may require voltages greater than can reasonably be achieved with a series combination of batteries. The final RF power amplifier in the transmitter may require a supply voltage greater than that provided by a reasonable combination of batteries. Thus, it is apparent that circuits within the mobile unit cannot operate off of raw battery voltage. Where a voltage step up is required, a linear regulator alone cannot be used.
The basic structure of a linear regulator is a pass transistor and a control circuit. The pass transistor receives the unregulated input voltage at the input to the regulator and outputs a regulated voltage. The control circuit utilizes a highly stable voltage reference diode to control the pass transistor. Differences in the input voltage and the regulated output voltage appear across the pass transistor. The input current to the linear regulator is essentially equal to the output current from the regulator. The voltage across the pass transistor multiplied by the current flowing through the pass transistor represents the power dissipated by the pass transistor.
Additionally, since the pass transistor may represent the only series element between the input and output terminals of the linear regulator, the linear regulator cannot generate a step up in the output voltage.
Switch Mode Power Supplies (SMPS) provide a solution to some of the power supply problems. A SMPS can generate a stable output voltage for a wide range of input voltages. Depending on the SMPS configuration, it is able to either step up or step down the input voltage if required. In multiple output SMPS configurations, the SMPS can provide both step up and step down voltages. In the most basic representation of a SMPS the input voltage is successively switched on and off to create an average output voltage. Of course merely switching the input voltage on and off does not create a regulated output voltage. The SMPS utilizes the electrical properties of additional inductor and capacitor elements to achieve a regulated output. In an inductor the current flowing through the inductor cannot be changed instantaneously. Similarly, in a capacitor the voltage across the capacitor cannot be changed instantaneously. The SMPS uses the energy storage functions of the inductor and capacitor to smooth the output of a SMPS from discontinuous on-off pulses to a smooth regulated voltage. When a transformer is used as the inductor element the SMPS is able to provide input-output voltage step up as well as input-output ground isolation. A Pulse Width Modulator (PWM) is commonly used as the control circuit governing the on-off switching times.
The advantage in using a SMPS over a linear regulator is not limited to the ability to step up the input voltage. The efficiency of the SMPS can reach 90% or greater. The SMPS efficiency is essentially constant over a wide range of input voltages. In contrast, the efficiency of a linear regulator decreases in proportion to increases in input voltage. However, because of the inherent current switching within a SMPS the output voltage is not as clean as the output voltage from a comparably filtered linear regulator. In a SMPS there is output voltage ripple as well as output voltage spikes at the switching frequency. The switching spikes and minimal level of output voltage ripple usually have no adverse effect on the load circuits. However, sensitive circuits that have reduced power supply noise rejection capabilities may be adversely effected. These circuits may include reference oscillators, frequency synthesizers, and RF amplifiers. As an example, a reference oscillator may exhibit spurious frequency components at f.sub.0 .+-.f.sub.s, where f.sub.0 represents the center frequency of the reference oscillator and f.sub.s represents the power supply switching frequency. A linear regulator can be used following the SMPS to provide the cleanest available voltage source to any sensitive circuits. Low Drop Out (LDO) regulators are used because of the small input/output voltage differential they require to provide full regulation. The low input/output voltage differential on the linear regulator is important because the voltage differential directly relates to the power dissipation within the regulator. Any power dissipation in the regulator corresponds to a decrease in overall efficiency. Therefore, to maximize efficiency the output voltage of the SMPS needs to be at the minimum level required to ensure full regulation from the LDO regulator. The output voltage of the SMPS can be set to a constant value by taking into account worst case SMPS output variation and worst case LDO regulator input voltage requirements. SMPS output voltage variation stems from a variety of factors including SMPS part differences, temperature, and load variations. Similarly, LDO regulator input voltage requirements stem from part differences, temperature, and load variation. If the output voltage of the SMPS is to be constant, the voltage will be non-optimal for a majority of conditions because of the need to account for cumulative worst case scenarios. The result of non-optimal SMPS output voltage is a decrease in overall efficiency. Reduced efficiency results in reduced battery life. The effect for the user is reduced mobile phone talk times and standby times. What is needed is a method for dynamically optimizing the output voltage of the SMPS to maximize efficiency.